<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
  <head>
    <meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
    <title>Store and Retrieve data or objects of complex types</title>
    <link rel="stylesheet" href="gettingStarted.css" type="text/css" />
    <meta name="generator" content="DocBook XSL Stylesheets V1.73.2" />
    <link rel="start" href="index.html" title="Berkeley DB Programmer's Reference Guide" />
    <link rel="up" href="stl.html" title="Chapter 8. Standard Template Library API" />
    <link rel="prev" href="stl_primitive_rw.html" title="Working with primitive types" />
    <link rel="next" href="stl_persistence.html" title="Dbstl persistence" />
  </head>
  <body>
    <div xmlns="" class="navheader">
      <div class="libver">
        <p>Library Version 12.1.6.2</p>
      </div>
      <table width="100%" summary="Navigation header">
        <tr>
          <th colspan="3" align="center">Store and Retrieve data or
        objects of complex types </th>
        </tr>
        <tr>
          <td width="20%" align="left"><a accesskey="p" href="stl_primitive_rw.html">Prev</a> </td>
          <th width="60%" align="center">Chapter 8. Standard Template Library API</th>
          <td width="20%" align="right"> <a accesskey="n" href="stl_persistence.html">Next</a></td>
        </tr>
      </table>
      <hr />
    </div>
    <div class="sect1" lang="en" xml:lang="en">
      <div class="titlepage">
        <div>
          <div>
            <h2 class="title" style="clear: both"><a id="stl_complex_rw"></a>Store and Retrieve data or
        objects of complex types </h2>
          </div>
        </div>
      </div>
      <div class="toc">
        <dl>
          <dt>
            <span class="sect2">
              <a href="stl_complex_rw.html#idm140526455409760">Storing varying length objects</a>
            </span>
          </dt>
          <dt>
            <span class="sect2">
              <a href="stl_complex_rw.html#idm140526455379088">Storing arbitrary sequences</a>
            </span>
          </dt>
          <dt>
            <span class="sect2">
              <a href="stl_complex_rw.html#idm140526455289392">Notes</a>
            </span>
          </dt>
        </dl>
      </div>
      <div class="sect2" lang="en" xml:lang="en">
        <div class="titlepage">
          <div>
            <div>
              <h3 class="title"><a id="idm140526455409760"></a>Storing varying length objects</h3>
            </div>
          </div>
        </div>
        <p>
            A structure like this:
        </p>
        <pre class="programlisting">class SMSMsg
{
public:
    size_t mysize;
    time_t when;
    size_t szmsg;
    int to;
    char msg[1];
};  </pre>
        <p>
            with a varying length string in <code class="literal">msg</code>
            cannot simply be stored in a
            <code class="literal">db_vector&lt;SMSMsg&gt;</code> without
            some configuration on your part. This is because, by
            default, dbstl uses the <span class="bold"><strong>sizeof()</strong></span> operator
            to get the size of an object and then <code class="function">memcpy()</code> to copy the
            object. This process is not suitable for this use-case as
            it will fail to capture the variable length string
            contained in <code class="literal">msg</code>.
        </p>
        <p> 
            There are currently two ways to store these kind of
            objects:
        </p>
        <div class="orderedlist">
          <ol type="1">
            <li>
              <p>
                    Register callback functions with dbstl that are
                    used to measure an object's size, and then
                    marshal/unmarshal the object.
                </p>
            </li>
            <li>
              <p>
                    Use a <code class="classname">DbstlDbt</code> wrapper
                    object. 
                </p>
            </li>
          </ol>
        </div>
        <div class="sect3" lang="en" xml:lang="en">
          <div class="titlepage">
            <div>
              <div>
                <h4 class="title"><a id="idm140526455412720"></a>Storing by marshaling objects</h4>
              </div>
            </div>
          </div>
          <p> 
                One way to store an object that contains
                variable-sized fields is to marshall all of the
                object's data into a single contiguous area in memory,
                and then store the contents of that buffer. This means
                that upon retrieval, the contents of the buffer must
                be unmarshalled. To do these things, you must register
                three callback functions: 
            </p>
          <div class="itemizedlist">
            <ul type="disc">
              <li>
                <p>
                        <code class="function">typedef void
                        (*ElemRstoreFunct)(T&amp; dest, const void
                        *srcdata);</code>
                    </p>
                <p>
                        This callback is used to unmarshal an
                        object, updating <span class="bold"><strong>dest</strong></span> using 
                        data found in <span class="bold"><strong>srcdata</strong></span>.
                        The data in <span class="bold"><strong>srcdata
                        </strong></span> contains the chunk of memory into
                        which the object was originally marshalled.
                        The default unmarshalling function simply
                        performs a cast (for example, <code class="literal">dest =
                        *((T*)srcdata)</code>), which assumes
                        the <span class="bold"><strong>srcdata</strong></span>
                        simply points to the memory layout of the
                        object.
                    </p>
              </li>
              <li>
                <p>
                        <code class="function">typedef size_t
                        (*ElemSizeFunct)(const T&amp;
                         elem);</code>
                    </p>
                <p> 
                        This callback returns the size in bytes
                        needed to store the <span class="bold"><strong>elem</strong></span> object.
                        By default this function simply uses <span class="bold"><strong>sizeof(elem)</strong></span> 
                        to determine the size of <span class="bold"><strong>elem</strong></span>.
                    </p>
              </li>
              <li>
                <p>
                        <code class="function">typedef void (*ElemCopyFunct)(void
                        *dest, const T&amp;elem);</code>
                    </p>
                <p> 
                        This callback is used to arrange all data
                        contained by <span class="bold"><strong>elem</strong></span> into
                        the chunk of memory to which <span class="bold"><strong>dest</strong></span>
                        refers. The size of <span class="bold"><strong>dest</strong></span> is set
                        by the <code class="function">ElemSizeFunct</code>
                        function, discussed above. The default
                        marshalling function simply uses
                        <code class="function">memcpy()</code> to copy
                        <span class="bold"><strong>elem</strong></span> to
                        <span class="bold"><strong>dest</strong></span>.
                    </p>
              </li>
            </ul>
          </div>
          <p> 
                The
                <code class="function">DbstlElemTraits&lt;SMSMsg&gt;::instance()-&gt;set_size_function()</code>,
                <code class="function">set_copy_function()</code> and
                <code class="function">set_restore_function()</code>
                methods are used to register these callback functions.
                If a callback is not registered, its default function
                is used. 
            </p>
          <p> 
                By providing non-default implementations of the
                callbacks described here, you can store objects of
                varying length and/or objects which do not reside in a
                continuous memory chunk — for example, objects
                containing a pointer which refers another object, or a
                string, and so forth. As a result,
                containers/iterators can manage variable length
                objects in the same as they would manage objects that
                reside in continuous chunks of memory and are of
                identical size. 
            </p>
        </div>
        <div class="sect3" lang="en" xml:lang="en">
          <div class="titlepage">
            <div>
              <div>
                <h4 class="title"><a id="idm140526455372688"></a>Using a <code class="classname">DbstlDbt</code> wrapper
                object</h4>
              </div>
            </div>
          </div>
          <p>
                To use a <code class="classname">DbstlDbt</code> wrapper
                object to store objects of variable length, a
                <code class="literal">db_vector&lt;DbstlDbt&gt;</code>
                container is used to store complex objects in a
                <code class="classname">db_vector</code>.
                <code class="classname">DbstlDbt</code> derives from DB
                C++ API's <code class="classname">Dbt</code>class, but can
                manage its referenced memory properly and release it
                upon destruction. The memory referenced by
                <code class="classname">DbstlDbt</code> objects is
                required to be allocated using the
                <code class="function">malloc()</code>/<code class="function">realloc()</code>
                functions from the standard C library.
            </p>
          <p> 
                Note that the use of
                <code class="classname">DbstlDbt</code> wrapper class is
                not ideal. It exists only to allow raw bytes of no
                specific type to be stored in a container. 
            </p>
          <p> 
                To store an <code class="classname">SMSMsg</code> object
                into a <code class="literal">db_vector&lt;DbstlDbt&gt;</code>
                container using a <code class="classname">DbstlDbt</code>
                object: 
            </p>
          <div class="orderedlist">
            <ol type="1">
              <li> 
                    Wrap the <code class="classname">SMSMSg</code>
                    object into a <code class="classname">DbstlDbt</code>
                    object, then marshal the SMSMsg object properly
                    into the memory chunk referenced by
                    <code class="methodname">DbstlDbt::data</code>. 
                </li>
              <li>
                    Store the <code class="classname">DbstlDbt</code>
                    object into a
                    <code class="literal">db_vector&lt;DbstlDbt&gt;</code>
                    container. The bytes in the memory chunk
                    referenced by the <code class="classname">DbstlDbt</code>
                    object's <span class="bold"><strong>data</strong></span>
                    member are stored in the
                    <code class="literal">db_vector&lt;DbstlDbt&gt;</code>
                    container.
                </li>
              <li>
                    Reading from the container returns a
                    <code class="classname">DbstlDbt</code> object whose
                    <span class="bold"><strong>data</strong></span> field
                    points to the <code class="classname">SMSMsg</code> object
                    located in a continuous chunk of memory. The
                    application needs to perform its own
                    unmarshalling.
                </li>
              <li>
                    The memory referenced by
                    <code class="literal">DbstlDbt::data</code> is freed
                    automatically, and so the application should not
                    attempt to free the memory.
                </li>
            </ol>
          </div>
          <p>
                <code class="classname">ElementHolder</code> should not be
                used to store objects of a class because it doesn't
                support access to object members using <span class="bold"><strong>(*iter).member</strong></span> or <span class="bold"><strong>iter-&gt;member</strong></span>
                expressions. In this case, the default
                <code class="literal">ElementRef&lt;ddt&gt;</code> is used
                automatically. 
            </p>
          <p>
                <code class="classname">ElementRef</code> inherits from
                <code class="classname">ddt</code>, which allows <span class="bold"><strong>*iter</strong></span> to return the object
                stored in the container. (Technically it is an
                <code class="classname">ElementRef&lt;ddt&gt;
                object</code>, whose "base class" part is the
                object you stored). There are a few data members and
                member functions in <code class="classname">ElementRef</code>,
                which all start with <code class="literal">_DB_STL_</code>. To
                avoid potential name clashes, applications should not
                use names prefixing <code class="literal">_DB_STL_</code> in
                classes whose instances may be stored into dbstl
                containers. 
            </p>
          <p> 
                Example code demonstrating this feature can be
                found in the
                <code class="methodname">StlAdvancedFeaturesExample::arbitrary_object_storage</code>
                method. 
            </p>
        </div>
      </div>
      <div class="sect2" lang="en" xml:lang="en">
        <div class="titlepage">
          <div>
            <div>
              <h3 class="title"><a id="idm140526455379088"></a>Storing arbitrary sequences</h3>
            </div>
          </div>
        </div>
        <p> 
            A sequence is a group of related objects, such as an
            array, a string, and so forth. You can store sequences of
            any structure using dbstl, so long as you implement and
            register the proper callback functions. By using these
            callbacks, each object in the sequence can be a complex
            object with data members that are all not stored in a
            continuous memory chunk. 
        </p>
        <p>
            Note that when using these callbacks, when you retrieve
            a stored sequence from the database, the entire sequence
            will reside in a single continuous block of memory with
            the same layout as that constructed by your sequence copy
            function. 
        </p>
        <p>
            For example, given a type RGB:
        </p>
        <pre class="programlisting">struct RGB{char r, g, b, bright;};  </pre>
        <p> 
            and an array of RGB objects, the following steps
            describe how to store an array into one key/data pair of a
            <code class="classname">db_map</code> container.
        </p>
        <div class="orderedlist">
          <ol type="1">
            <li> 
                Use a <code class="classname">db_map&lt;int, RGB *,
                ElementHolder&lt;RGB *&gt; &gt;</code>
                container. 
            </li>
            <li>
              <p>
                    Define two functions. The first returns the
                    number of objects in a sequence, the second that
                    copies objects from a sequence to a defined
                    destination in memory: 
                </p>
              <pre class="programlisting">typedef size_t (*SequenceLenFunct)(const RGB*);  </pre>
              <p>
                    and 
                </p>
              <pre class="programlisting"> typedef void (*SequenceCopyFunct)(RGB*dest, const RGB*src);  </pre>
            </li>
            <li> 
                Call
                DbstlElemTraits&lt;RGB&gt;::set_sequence_len_function()/set_sequence_copy_function()
                to register them as callbacks.
            </li>
          </ol>
        </div>
        <div class="sect3" lang="en" xml:lang="en">
          <div class="titlepage">
            <div>
              <div>
                <h4 class="title"><a id="idm140526455361216"></a>The <code class="function">SequenceLenFunct</code>
                function</h4>
              </div>
            </div>
          </div>
          <pre class="programlisting">typedef size_t (*SequenceLenFunct)(const RGB*);  </pre>
          <p>
                A <code class="function">SequenceLenFunct</code> function
                returns the number of objects in a sequence. It is
                called when inserting into or reading from the
                database, so there must be enough information in the
                sequence itself to enable the
                <code class="function">SequenceLenFunct</code> function to
                tell how many objects the sequence contains. The
                <code class="literal">char*</code> and
                <code class="literal">wchar_t*</code> strings use a
                <code class="literal">'\0'</code> special character to do
                this. For example, RGB(0, 0, 0, 0) could be used to
                denote the end of the sequence. Note that for your
                implementation of this callback, you are not required
                to use a trailing object with a special value like
                <code class="literal">'\0'</code> or <code class="literal">RGB(0, 0, 0,
                 0)</code> to denote the end of the sequence.
                You are free to use what mechanism you want in your
                <code class="function">SequenceLenFunct</code> function
                implementation to figure out the length of the
                sequence. 
            </p>
        </div>
        <div class="sect3" lang="en" xml:lang="en">
          <div class="titlepage">
            <div>
              <div>
                <h4 class="title"><a id="idm140526455298672"></a>The <code class="function">SequenceCopyFunct</code> function</h4>
              </div>
            </div>
          </div>
          <pre class="programlisting"> typedef void (*SequenceCopyFunct)(RGB*dest, const RGB*src); </pre>
          <p>
                <code class="function">SequenceCopyFunct</code> copies objects
                from the sequence <span class="bold"><strong>src</strong></span>
                into memory chunk <span class="bold"><strong>dest</strong></span>. If the
                objects in the sequence do not reside in a continuous memory chunk, this
                function must marshal each object in the sequence into
                the <span class="bold"><strong>dest</strong></span> memory
                chunk. 
            </p>
          <p> 
                The sequence objects will reside in the continuous
                memory chunk referred to by <span class="bold"><strong>dest</strong></span>, which
                has been sized by
                <code class="classname">SequenceLenFunct</code> and
                <code class="classname">ElemSizeFunct</code> if available
                (which is when objects in the sequence are of varying
                lengths). <code class="classname">ElemSizeFunct</code>
                function is not needed in this example because
                <span class="bold"><strong>RGB</strong></span> is a simple
                fixed length type, the <code class="literal">sizeof()</code>
                operator is sufficient to return the size of the
                sequence.
            </p>
        </div>
      </div>
      <div class="sect2" lang="en" xml:lang="en">
        <div class="titlepage">
          <div>
            <div>
              <h3 class="title"><a id="idm140526455289392"></a>Notes</h3>
            </div>
          </div>
        </div>
        <div class="itemizedlist">
          <ul type="disc">
            <li>
              <p> 
                    The get and set functions of this class are not
                    protected by any mutexes. When using multiple
                    threads to access the function pointers, the
                    callback functions must be registered to the
                    singleton of this class before any retrieval of
                    the callback function pointers. Isolation may also
                    be required among multiple threads. The best way
                    is to register all callback function pointers in a
                    single thread before making use of the any
                    containers. 
                </p>
            </li>
            <li>
              <p>
                    If objects in a sequence are not of identical
                    sizes, or are not located in a consecutive chunk
                    of memory, you also need to implement and register
                    the
                    <code class="function">DbstlElemTraits&lt;&gt;::ElemSizeFunct</code>
                    callback function to measure the size of each
                    object. When this function is registered, it is
                    also used when allocating memory space. </p>
              <p>
                    There is example code demonstrating the use
                    this feature in the
                    <code class="methodname">StlAdvancedFeaturesExample::arbitray_sequence_storage()</code>
                    method. 
                </p>
            </li>
            <li>
              <p>
                    A consequence of this dbstl feature is that you
                    can not store a pointer value directly because
                    dbstl will think it is a sequence head pointer.
                    Instead, you need to convert the pointer into a
                    <code class="literal">long</code> and then store it into
                    a <code class="literal">long</code> container. And please
                    note that pointer values are probably meaningless
                    if the stored value is to be used across different
                    application run times.
                </p>
            </li>
          </ul>
        </div>
      </div>
    </div>
    <div class="navfooter">
      <hr />
      <table width="100%" summary="Navigation footer">
        <tr>
          <td width="40%" align="left"><a accesskey="p" href="stl_primitive_rw.html">Prev</a> </td>
          <td width="20%" align="center">
            <a accesskey="u" href="stl.html">Up</a>
          </td>
          <td width="40%" align="right"> <a accesskey="n" href="stl_persistence.html">Next</a></td>
        </tr>
        <tr>
          <td width="40%" align="left" valign="top">Working with primitive types  </td>
          <td width="20%" align="center">
            <a accesskey="h" href="index.html">Home</a>
          </td>
          <td width="40%" align="right" valign="top"> Dbstl persistence</td>
        </tr>
      </table>
    </div>
  </body>
</html>
